1. Field of the Invention
This invention concerns a power amplifier circuit having a stand-by mode and an operating mode.
2. Description of the Prior Art
A power amplifier circuit having a stand-by mode to reduce power consumption is known. In the stand-by mode, no current is supplied to a power amplifier circuit. FIG. 1 shows a conventional power amplifier circuit provided with a stand-by circuit to achieve a stand-by mode. The power amplifier circuit of FIG. 1 comprises an output stage 3, including bipolar transistors Q1 and Q2. The transistors Q1 and Q2 are connected in totem-pole configuration, and are driven by an output of an amplifier 1. The amplifier 1 is biased with power sources VCC(+) and VEE(-). An output of the output stage 3 is supplied to a load 5, e.g., an audio speaker.
The numeral 7 designates a bias circuit. The bias circuit 7 provides a bias current to the amplifier 1. In this circuit, an NPN transistor Q4, a PNP transistor Q3, the bias circuit 7 and the amplifier 1 constitute the stand-by circuit. The base electrode of the transistor Q4 is connected to a stand-by terminal 9, and the emitter electrode thereof is connected to the power terminal VEE. The PNP transistor Q3, the emitter electrode of which is connected to the power terminal VCC, supplies current to the bias circuit 7 and the amplifier 1. In the stand-by mode, the level of the signal at the stand-by mode terminal 9 is set to a low level. In this mode, the NPN bipolar transistor Q4 and the PNP bipolar transistor Q3 are in the OFF state, and the current supplied by the PNP transistor Q3 to the bias circuit 7 and the amplifier 1 is turned off. In this way, a stand-by state is achieved. With this stand-by mode or stand-by circuit, it is possible to turn the amplifier circuit ON or OFF without a large capacity switch on the power supplying line. Thus, the cost for producing the circuit can be reduced. Furthermore, by setting the threshold voltage of the stand-by circuit to a proper voltage, it is possible to control the stand-by circuit by a logic controller, micro-computer, etc. (not illustrated). Thus, use of the power amplifier circuit in an audio apparatus, for example as a muting circuit, becomes easier.
However, in this circuit, as the amplifier circuit changes from the operation state to the stand by state, at the moment when the stand-by mode is selected, a shock noise is generated. This shock noise sometimes causes an unpleasant noise when the circuit is used in a muting circuit.
FIG. 2 shows a conventional amplifier circuit designed to improve on the above defect. In this circuit, a timer circuit 10 is provided. Namely, the collector electrode of the transistor Q3 is connected to the power terminal VEE through resistors R1 and R2. a capacitor C1 is connected in parallel to the registor R2 to form a timer circuit 10. The bias circuit 7 is biased by the timer circuit 10, and the operation thereof is controlled by the timer circuit 10. Other parts designated by the same numeral as in FIG. 1 show corresponding parts.
In this circuit, when the stand-by mode is selected, the potential at the terminal 9 is set to a low level. In this condition, the transistors Q4 and Q3 are in the OFF state. Thus, a charge stored in the capacitor C1 during the operation mode is gradually discharged through the resistor R2. Therefore, the bias to the bias circuit 7 by the timer circuit 10 is gradually reduced. When the bias potential of the timer circuit 10 drops below a threshold level of the bias circuit 7, the bias circuit 7 changes to the OFF state and stops biasing the amplifier 1. In this way, the change from the operating state to the stand-by state is performed gradually, in accordance with the time constant C1.R2. By changing the amplifier 1 from the operation state to the stand-by state gradually, the shock noise is eliminated. However, in this circuit, as the amplifier circuit 1 changes to the stand-by state gradually, noise from a pre-amplifier (not shown) is amplified by the amplifier 1, during the change from the operation state to the stand-by state. Therefore, when the amplifier circuit is used in a muting circuit, a brief sound may be output during the muting operation.